This Marie Skłodowska Curie Action (MSCA) has investigated the interplay of composition, structure, and mechanical properties of zeolitic imidazolate framework (ZIF) glasses, ultimately paving the way for the development of more fracture-resistant materials without compromising the ability to fabricate bulk samples.
Among metal-organic frameworks (MOFs), ZIFs stand out as a crucial subset due to their exceptional glass-forming ability. This has enabled the vitrification of various ZIF crystals into glasses, with some of them maintaining the porosity of their parent crystals. Consequently, the emergence of ZIF glasses has attracted significant attention, primarily due to their distinct physical and chemical characteristics compared to traditional glass families. Despite the recent progress in understanding the structure-property correlations in ZIF glasses, there are still many unanswered questions regarding their mechanical properties, greatly hindering their potential engineering and functional applications.
In detail, the project aimed at exploring the structure vs. mechanical property relationship of ZIF glasses. To this end, the objectives of this MSCA have been achieved through the following key factors:
(a) Enable prediction and control of the ZIF glass structure: We aimed at producing ZIF glasses with different structures. The glass structures were controlled by compositional design combined with the change in the synthesis conditions. First, we investigated the modification effect of water on the glass forming behaviour of MOFs by combining experiments and ab initio simulations. Second, we investigated the structure change of ZIF-4 (the first ZIF to be melt-quenched to a glassy state) upon ion irradiation. We observed irradiation-induced disordering of ZIF-4 crystal and glass, which resulted in to the formation of a “forbidden state,” i.e. a state that is inaccessible through simple thermal treatment such as heating and cooling. Third, we investigated the structure of ZIF-4 upon hot compression which could potentially be a method to improve the mechanical properties.
(b) Understand the fracture behavior and mechanical properties of ZIF glasses with different structures: We systematically investigated the fracture behavior and properties of ZIF glasses with different structures. To this end, fracture simulations were carried out on two series of ZIF glasses: (i) ZIF-4 subjected to different degrees of irradiation, and (ii) ZIF-62 with different compositions, i.e. Zn(Im)2-x(BIm)x, where x = 0, 0.1 0.25 0.35 0.5 0.65 0.8. 1.0. The outcome of this object enables the establishment of the correlation of structure and mechanical properties in ZIF glasses.
(c) Reveal the origin of the low fracture toughness of ZIF glasses and identify suitable routes for improvement: The structure vs. mechanical property relationship of ZIF glasses was then explored by identifying the structure fingerprint responsible for the fracture patterns. To identify the atomistic origin of the fracture behavior of ZIF glasses, a new deep learning-based force field (DLFF) has been developed in this project. Furthermore, the influence of the structural parameters on the mechanical properties was clarified to facilitate the design of ZIF glasses with tailored fracture patterns.
The overall findings will help to enable the rational design of MOF glass materials with the ability to sustain higher stress and plastically deform without fracture. The effect of glass composition and post-processing routes on the atomic structure and microstructure has been identified, including irradiation treatment and control of the thermal and pressure history. The influence of the structural parameters on the mechanical properties has been clarified to facilitate the design of ZIF glasses with tailored fracture patterns.
